There are many homogenous or heterogenous chemical reactions involving liquid and/or gas vapor phases that benefit from the intimate mixing of the reactants in the reaction zone.
This intimate mixing is usually supplied by a tubular reactor. These reactors consist of a long conduit into which the reactants are injected. Mixing of the reactants occur as they flow down the conduit. The design requirements for these reactors include the variables of temperature, degree of mixing and residence time. Direct or indirect heat transfer may be employed to control temperature conditions within the tubular reactor. For example it is known that such reactors may be externally jacketed to circulate a heat exchange medium on the outside surface of an extended reaction conduit and thereby provide indirect heating or cooling over the entire external surface of the reactor.
The primary variables influencing the design of the tubular reactor are degree of mixing and residence time. The length of the conduit or pipe is usually sized to control residence time. The degree of mixing is largely a function of the flow regime within the conduit. In open tubular reactors the diameter largely controls the flow regime therein. Thus optimal velocity for tubular reactions is established when the pipe diameter correctly keeps the flow in the desired flow regime with a pipe or conduit length that is fight to reach the proper residence time for the reaction.
The requirements for mixing and residence time are not always fully compatible, and therefore, the diameter of a tubular reactor may represent a compromise in optimum values to control mixing and residence time. In addition, many tubular reactors require very long pipe lengths at high velocities to achieve the necessary mixing. One means of overcoming the incompatibility in the flow regime or residence time and long length requirements is the use of internal mixers within a tubular reactor or other reaction zone. Internal mixing devices include stirred reactors and static mixers.
In some cases, tubular reactors are also unable to provide the intensity of the mixing that may be important for certain reactions. In order to overcome mass transfer limitations, many reactions that require intimate mixing of reactants also require the mixing be accomplished with a high degree of shear forces between the fluids. The high shear forces create the necessary phase dispersion to overcome mass transfer limitations inherent in the fluids and to provide the contacting necessary for precise reaction control.
Stirred tank reactors in many cases may provide the necessary shear forces to eliminate mass transfer limitations. However, stirred tank reactors often provide unwanted areas of stagnation that allow variations in residence time and degrade the products obtained from certain reactions. In addition, the mechanical elements of stirred tank reactors may prove troublesome. When operating at high pressure, impeller shaft seal leakage is particularly difficult to prevent.
Static mixers are commonly used to supply additional mixing energy to the reactor instead of mechanical stirred reactors. These types of static mixers include simple static mixers, fluidic mixers and vortex mixers. Simple static mixers are effective in forming and dispersing gas bubbles in a statistical distribution. However, the static mixers will not go beyond dispersal of bubbles and provide the shear force that is often necessary for thorough mixing. The fluidic mixers and vortex mixers both provide plug flow conditions that overcome the stagnation encountered with many stirred tank reactors. These types of mixers use pressure drop or pressure pulsations to add mixing energy to the reactors and create the desired degree of mixing. The fluidic mixing devices require high pressure pulsations to give the correct phase dispersion distribution within the reactor. Vortex mixtures use a high pressure drop to create a vortex that blends fluids at high intensity to keep the reactants mixed. Providing high pressure pulsations for fluidic mixing increases the mechanical complexity of the system. Both the vortex mixers and fluidic mixers have substantial energy requirements associated with providing the mixing energy needed to impose high shear conditions on the reactants passing through the reactor. The requirement of high shear forcing for mixing can also cause static mixers to create segregated bubbles within the reactant stream. These segregated bubbles again prevent the creation of the desired uniform mixtures of well-dispersed fluids for passage through the reaction zone.
U.S. Pat. No. 5,017,343 issued to Cetinkaya shows a mixing device that provides mixing of liquid and gaseous streams for the contacting of the mixed components with a fluidized stream of solid material.